Power Sources for Welding Processes | Metals | Industries | Metallurgy

In this article we will discuss about the power sources for welding processes.

Solid State Power Sources for Welding:

Solid state power sources in comparison to conventional power sources are considered to be more efficient, have fast response characteristics and are must for some special jobs, particularly the applications performed with the aid of robots. In pulsed arc welding, a high power spray of small droplets is produced and this can be directed in any direction.

The ratio of the pulse current and the maintenance arc current has to be adjusted to meet varying conditions of material thickness and attitude, which is possible only with solid-state power sources.

By using square wave AC power source it is possible to weld a narrow groove (with included angle of 2-4°) in thick materials, for which V-groove (with included angle of 30°— 60° is required with conventional DC sources) thus resulting in considerable saving of filler metal.

Power Sources for Arc Welding Processes:

Arc welding requires special electric power of low voltage and high current with desirable characteristic to produce and sustain an arc capable of making a good weld.

Power sources may be rotating type or static type (transformer or rectifier). Electronic control with feedback (thyristor control) is popular. Output could be AC, DC or both. Characteristics may be constant current, constant voltage of combination of both. Source should be capable of delivering desired current at specified duty.

The basic features of power source are:

(i) Static Characteristic Curves:

Static characteristic curves, i.e. relationship between load voltage and load current under various load conditions. Curve C in Fig. 9.49 is drooping curve with constant current. Small variations in voltage do not significantly change the output current. It is suitable for MMAW/SMAV/GTAW processes.

In these sources current is more or less constant even with small variations in arc length which is unavoidable even with skilled worker. Thus weld quality is consistent and current does not exceed heavily even at arc strike and accidental situations. Curve B is relatively flat (constant voltage) source. Such machines are designed on DC and not suitable with AC output.

This source is used in semi­automatic and automatic processes like MIG/C0₂ welding, SAW, flux cored arc welding. This process is self-regulating to control melting rate and feeding rate. Voltage drop across arc is proportional to arc length. A small change in arc voltage results in a very large change in current and increases melting rate and quickly restores the arc length to normal.

By selecting a correct combination of voltage and wire feed speed, a stable metal transfer can be obtained to get good quality welds.

(ii) Open Circuit Voltage OCV:

It is the voltage across output terminal of the power source when there is no welding. In case of constant current source, OCV is high and ensures easy arc starting. Higher the OCV, better is the arc stability. IS : 4559 restricts OCV to 100 V to avoid danger of shock to welder.

(iii) Dynamic Characteristic:

These determine the relation between voltage and current under changing load conditions. Welding arc is never steady and is subjected to severe and rapid fluctuations due to constant small varia­tions in arc length, voltage and current. A power source with good dynamic characteristics enables a very stable and smooth arc even with transients.

(iv) Rating and Duty Cycle:

Power sources are speci­fied as output current at specific duty cycle which is defined as the percentage of a five-minutes interval that it operates at a given current setting. A 70% duty cycle means arc is in action for 3.5 minutes out of a 5-minute time period. Duty is based on successive 5-minutes intervals. 60% duty cycle is taken as standard.

In addition various insulation classes and the corresponding maximum permissible temperature are specified in IS : 4559.

Power factor of power source should be high to avoid wastage and efficient utilisation of power.

Generators are designed for dc welding and alternators provide ac output which can be rectified to provide dc output.

Solid state devices (silicon controlled rectifier- thyristor) with special characteristics can be used to directly control welding power by altering the welding current or voltage wave form.

In case of hazardous conditions (like very high/low ambient, high altitude, excessive humidity, dust/vapours, vibration/shock, corrosive environment, etc, welding power supply sources must be specifically designed.

Waveform Controlled Power Sources for Welding:

These power sources do the thinking and actually control the electrode current throughout the welding cycle. These make best use of sophistication of computers and power of inverter technology. These manipulate waveforms of power supply by sophisticated internal control software, coordinating machine control variables automatically and thus control welding output to improve overall weld quality.

Quality of electrodes can be estimated by analysis of welding current and voltage process modelling. A waveform is a representation of the dynamic, ever- changing output response to an arc welding power source to the actions of the electric arc itself. Mode switches offer the multiple waveform option.

Waveform control provides an extremely stable arc that easily handles out-of-position welds. It uses lesser power, offers cleaner shop environment because of reduced welding fume, radiation and spatter, lower training costs, etc.

Inserting Versatility into Power Supply Systems:

With introduction of chips into power supply systems for welding processes, the power supply systems have become so versatile that there is no need to select different equipment for different welding processes. Systems are now available which can support a number of processes and weld procedures.

Options are available to develop new welding programmes. A single machine can thus adopt to a wide range of applications, different materials, enable welding in different positions. It enables integration of technologies, products, processes and applications.

Such equipment enables optimised metal transfer modes reducing spatter and improving arc stability and extend the operating range of the welding process. Higher productivity is achieved due to higher deposition rates with less heat input. Such equipment affords fast, smart, efficient, quality operations enabling to stand quality competition.

It is possible to adjust output parameters for given welding conditions and joint design to meet weld specifications and production rates. It thus enables to choose a weld programme from a pre-defined set of programmes and to manipulate the parameters (wire feed speed, voltage, current, arc control etc.) of that programme to best fit the given application.

The transfer of metal from the electrode to weld puddle is governed by several forces like surface tension, welding current and the magnetic field produced by it, aerodynamic forces, etc. The metal transfer within the arc can be controlled by balancing the forces of surface tension and the welding current.

Several sensors work in such a harsh environment and provide electrical signals to high speed controllers to make appropriate changes to the current waveform to achieve desired results in the form of consistent high quality welds.